1. Field of the Invention
The present invention relates to a voltage controlled oscillator using a resonant phenomenon of a parallel LC tank circuit. More particularly, the invention relates to a voltage controlled oscillator that is equipped with a capacitance switches and capable of changing an oscillation frequency in steps.
2. Description of the Related Art
Recently, an LC-voltage controlled oscillator (LC-VCO), which uses the resonant phenomenon of the parallel LC tank circuit, has been used as the local oscillator (LC) of a phase locked loop (PLL) circuit used for frequency multiplication and phase lock. In the LC-VCO, an inductor and a variable capacitor are mutually connected in parallel to form the parallel LC tank circuit, and the resonant phenomenon of the parallel LC tank circuit oscillates an alternating-current signal whose frequency is a resonant frequency. The resonant frequency is a frequency at which the impedance of the parallel LC tank circuit becomes infinite, and the resonant phenomenon is a phenomenon where electric current alternately flows in the inductor and the variable capacitor in the parallel LC circuit. Further, a varactor device or the like is used as the variable capacitor, whose capacitance varies according to a control voltage to be applied. Assuming the inductance of the inductor and the capacitance of the variable capacitors are L and C, respectively, the resonant frequency f is given by the following expression 1. According to the following expression 1, it is made clear that the resonant frequency f reduces when the capacitance C of the variable capacitors is increased.
                    f        =                  1                      2            ⁢            π            ⁢                                                  ⁢                                          L                ⁢                                                                  ⁢                C                                                                        [Expression  1]            
Comparing the LC-VCO with a conventional VCO using a ring oscillator or the like, the LC-VCO has advantages shown below. Firstly, the LC-VCO has less noise than the conventional VCO using the ring oscillator or the like. This is due to the small number of transistors that cause noise because the LC-VCO uses the resonance of the parallel LC tank circuit as a basic principle. Therefore, the LC-VCO is preferable for high-speed optical communication, cell phones, wireless LAN and the like. Secondary, since the LC-VCO uses the resonance of the LC circuit as the basic principle, it is easier to obtain a high oscillation frequency than the VCO that consists of transistors and uses logic gate delay. Thirdly, the LC-VCO has the small variable width of oscillation frequency corresponding to the control voltage. For this reason, tuning sensitivity is low and oscillation frequency variation caused by the variation of the control voltage is small, which consequently leads to low noise.
On the other hand, a drawback of the LC-VCO is the above-described low tuning sensitivity. As described, although the low tuning sensitivity works advantageously to noise, the variable width of the oscillation frequency becomes small, and thus designing of the LC-VCO that achieves a desired oscillation frequency is difficult.
To overcome the drawback, an LC-VCO provided with the capacitance switches is suggested as shown in the document ‘A. Kral et al., “RF-CMOS Oscillators with Switched Tuning”, IEEE Custom Integrated Circuits Conf., pp. 555–558, 1998’ for example. FIGS. 1A to 1C are equivalent circuit diagrams showing the LC-VCO provided with conventional capacitance switches, where FIG. 1A, FIG. 1B and FIG. 1C respectively show an equivalent circuit diagram showing the entire LC-VCO, an equivalent circuit diagram showing the capacitance switch section in an OFF state, and an equivalent circuit diagram showing the capacitance switch section in an ON state. Further, FIGS. 2A and 2B are graphs showing the variable width of the oscillation frequency taking the control voltage on the axis of abscissas and the oscillation frequency on the axis of ordinate, where FIG. 2A and FIG. 2B respectively show the variable range of the oscillation frequency when the capacitance switch section is not used, and the variable range of the oscillation frequency when the capacitance switch section is used.
As shown in FIG. 1A, the conventional LC-VCO 101 is connected to power source potential wiring VCC and ground potential wiring GND. In the LC-VCO 101, a negative resistance section 2, an LC circuit section 104 and a negative resistance section 3 are arranged in this order from the power source potential wiring VCC toward the ground potential wiring GND.
P-channel transistors 5, 6 are provided in the negative resistance section 2. One of the source/drain of the P-channel transistor 5 is connected to the power source potential wiring VCC, the other one is connected to an output terminal 7 of the LC circuit section 104, and a gate is connected to an output terminal 8. One of the source/drain of the P-channel transistor 6 is connected to the power source potential wiring VCC, the other one is connected to an output terminal 8 of the LC circuit section 104, and the gate is connected to an output terminal 7.
In the LC circuit section 104, an inductor 9 is provided between the output terminals 7, 8. Furthermore, the variable capacitors 10, 11 are provided in series between the output terminals 7, 8 parallelly with the inductor 9. The variable capacitors 10, 11 are the capacitors whose capacitance varies according to the control voltage to be input, which is the varactor device, specifically.
Moreover, a capacitance switch section 116 is provided for the LC circuit section 104, and capacitors 12, 13 and switches 14, 15 are provided for the capacitance switch section 116. The output terminal 7 is connected to one electrode of the capacitor 12, the other electrode of the capacitor 12 is connected to one terminal of the switch 14, and the other terminal of the switch 14 is connected to a ground electrode. In the same manner, the output terminal 8 is connected to the ground electrode via the capacitor 13 and the switch 15. The switches 14, 15 essentially consist of NMOS (N-type Metal Oxide Semiconductor) transistors. Although FIG. 1A shows only one capacitance switch section 116, a plurality of the capacitance switch sections 116 may be provided and connected between the output terminals 7, 8 parallelly with each other.
N-channel transistors 17, 18 are provided in the negative resistance section 3. One of the source/drain of the N-channel transistor 17 is connected to the output terminal 7 of the LC circuit section 104, the other one is connected to the ground potential wiring GND, and the gate is connected to the output terminal 8. Further, one of the source/drain of the N-channel transistor 18 is connected to the output terminal 8, the other one is connected to the ground potential wiring GND, and the gate is connected to the output terminal 7.
Next, the operation of the conventional LC-VCO 101 will be described. For example, when an electrical stimulation is applied to the LC circuit section 104 by connecting the LC-VCO 101 to the power source potential wiring VCC and ground potential wiring GND, or the like, the LC circuit section 104 oscillates a high frequency (HF) having the resonant frequency from the output terminals 7, 8. At this point, signals output form the output terminals 7, 8 are complementary signals.
However, loss by parasitic resistance is caused and oscillation stops in the end in the case of the only LC circuit section 104. Given this factor, a positive power source potential is applied to the power source potential wiring VCC and a ground potential is applied to the ground potential wiring GND to supply an electric current and provide the negative resistance sections 2, 3 to/for the LC-VCO 101, and thus it is possible to permanently oscillate resonant waves in the LC circuit section 104. Specifically, when the output terminal 7 is low and the output terminal 8 is high, for example, the P-channel transistor 5 is turned OFF and the N-channel transistor 17 is turned ON. Consequently, the ground potential is applied to the output terminal 7. Further, since the P-channel transistor 6 is turned ON and the N-channel transistor 18 is turned OFF, the power source potential is applied to the output terminal 8. In the same manner, when the output terminal 7 is high and the output terminal 8 is low, the power source potential is applied to the output terminal 7 and the ground potential is applied to the output terminal 8. Thus, the oscillation from the output terminals 7, 8 continues without attenuation.
Then, by changing the control voltage applied to the variable capacitors 10, 11, the capacitance of the variable capacitors 10, 11 linearly varies. As a result, as shown in FIG. 2A, the resonant frequency of the LC circuit section 104 varies according the control voltage so that it is possible to vary the frequency of high frequency (HF) that the LC-VCO 101 oscillates. Range A shown in FIG. 2A is the variable range of the oscillation frequency.
Furthermore, the capacitance of the entire capacitance switch section 116 varies by switching the switches 14, 15. As shown in FIG. 1B, since the switch 14 works as a capacitance and is connected to the capacitor 12 in series when the switch 14 is turned OFF, the total capacitance of the capacitor 12 and the switch 14 becomes relatively small. On the other hand, as shown in FIG. 1C, when the switch 14 is turned ON, the switch 14 works as a resistance and the total capacitance of the capacitor 12 and the switch 14 becomes relatively large. Consequently, when the switch 14 is turned OFF, the capacitance of the entire LC circuit section 104 becomes small, and thus increasing the oscillation frequency according to the above-described expression 1. Further, when the switch 14 is turned ON, the capacitance of the entire LC circuit section 104 becomes large, and thus decreasing the oscillation frequency. Accordingly, by opening/closing the switches 14, 15, it is possible to discontinuously vary the oscillation frequency. Further, it is possible to vary the oscillation frequency in steps if a plurality of the capacitance switch sections 116 are provided and individually opened/closed.
Consequently, as shown in FIG. 2B, the oscillation frequency is made to vary in steps by the capacitance switch section 116 and the control voltage of the variable capacitors 10, 11 is changed to continuously vary the oscillation frequency, so that the variable width of the oscillation frequency can be increased while the tuning sensitivity is maintained low and the oscillation frequency variation due to the variation of control voltage is inhibited, comparing to the case where the capacitance switch section 116 is not provided. Range B shown in FIG. 2B is the variable range of the oscillation frequency. Further, an oscillation frequency band can be changed by providing the capacitance switch section 116, which readily deal with a plurality of frequencies demanded in the error correction or the like in a communication system.
However, the above-described prior art show the following problems. As described, the N-channel transistor such as the NMOS transistor is used for the switches 14, 15 shown in FIG. 1A. Unlike an ideal switch, since parasitic component remains in the switches formed by such transistor in both ON state and OFF state, it is impossible to completely connect or disconnect the capacitors 12, 13 to the ground electrode. Specifically, when the switches 14, 15 are in the ON state, the switches 14, 15 work as a resistance, and they work as a capacitance in the OFF state. Therefore, when the switches 14, 15 are in the ON state, the electrode potential on the ground electrode side in the capacitors 12, 13 does not become completely the ground potential, but is affected by the potential of the output terminals 7, 8 and alternating current component remains. As a result, the effective voltage between the electrodes of the capacitors 12, 13 becomes small, and thus decreasing the effective capacitance of the capacitors 12, 13. Accordingly, an effect to reduce the oscillation frequency of the LC-VCO 101 is insufficient even if the switches 14, 15 are turned ON, and the variable width of the oscillation frequency becomes small.
Note that the resistance of the switches 14, 15 in the ON state becomes small if the channel width of the transistors that consist the switches 14, 15 is increased, and the behavior of the LC-VCO 101 becomes almost an ideal state. However, there is a problem that the size of the entire LC-VCO 101 becomes large when the transistors are larger.